1. Field of the Invention
This invention relates to metal welding using high energy beams such as laser beams and electron beams. More particularly, it pertains to seam welding especially joints that connect and seal cover portions of a torque converter.
2. Description of the Prior Art
Conventionally, torque converters for automotive transmissions include two cover portions, an impeller housing and converter cover, joined by fillet welding along a seam where the covers overlap. The weld is made by heat produced by an electric arc welding gun operated in an inert gas atmosphere using metal inert gas (MIG) weld techniques. Filler wire, melted and fused to molten metal of the cover and housing, is added to the parent metal and provides material for the fillet.
Before welding, the cover and housing are formed from high strength, low alloy steel to the desired shape in a press having an enormous force capacity--perhaps 3500 tons or more. In the process of forming the cover and housing with great forming pressures produced in the press, residual stresses of substantial magnitude are induced and remain in the parts after forming. Conventional welding, such as MIG welding, causes a large temperature spreading over a large area adjacent the seam that connects the cover and housing. The high temperature releases the residual stresses induced while forming and causes distortion of the cover and housing as the residual stresses are relieved by the heat of welding.
In a motor vehicle, the assembly formed by welding the cover and housing rotates at the high speed of the engine that drives the vehicle. Acceptable performance in this environment requires an accurate, dynamic balance of the torque converter about the axis of rotation. To assure satisfactory balance after forming and welding the torque converter, weights are riveted to the outer surface of the cover to restore the balance of the torque converter assembly within an acceptable range of balance established by performance criteria. Both the distortion that results from overcoming the residual stresses and the addition of the filler wire required by conventional weld processes operate to disturb the initial balance of the torque converter and require greater correcting balance weight and more attention to the dynamic balance than would be required if these undesirable effects of the conventional weld process could be eliminated.
Necessary manufacturing and assembly tolerances required to produce the impeller housing and converter cover and to assemble these with a lap joint cause a gap between the inner surface of the housing and the outer surface of the cover in the region where they overlap and a seam is to be formed. The location of the inner surface of the outer member at the overlap can vary over a tolerance range for that member, and the location of the outer surface of the inner member at the overlap can vary over a tolerance range for that member. Consequently, the width of the gap will vary in accordance with the combination of tolerance ranges associated with the cover and housing to be joined by welding. Where electric arc welding is used, the presence of the gap presents no serious difficulty. The large amount of heat produced, the broad zone of elevated temperature that results and the use of filler wire provide a sufficient amount of molten metal to span the gap and make an effective joint.
However, where a welding beam, focused on a small area or at a point, is used to make the weld and no filler wire is used, as in the present invention, the presence of a gap presents a serious difficulty. Yet a gap before welding is essential because of the tolerances required to assure assembly of the cover and housing.
The assembly comprises a shell containing transmission fluid under pressure. Therefore, the seam weld between the cover and housing must produce a leak-free seal with respect to hydraulic fluid. Also, the rate at which the weld is made should be as fast as possible consistent with these requirements.